The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Compression-ignition engines, e.g., diesel engines, offer benefits including improved fuel economy. Manufacturers of vehicles, including passenger cars, commercial trucks, construction, and agricultural tractors, must meet stringent emissions requirements to market their products.
Controlling ignition timing, i.e., retarding or advancing timing, on a diesel engine is a valuable control option, in order to achieve low smoke emissions. For example, it has been demonstrated that in a system operating in lean air/fuel ratio in a compression-ignition mode with a premixed cylinder charge (i.e., controlled auto-ignition or homogeneous-charge compression-ignition, or HCCI), it is necessary to complete the injection of all fuel before ignition occurs, in order to achieve benefits related to lowered particulate matter (PM) emissions.
In an engine operating in auto-ignition mode, combustion of a cylinder charge is flameless, and spontaneously occurs throughout the entire combustion chamber volume. The homogeneously mixed cylinder charge auto-ignites as the cylinder charge is compressed and its temperature increases. The ignition timing of auto-ignited combustion depends on initial cylinder charge conditions including, primarily, temperature, pressure, and composition of the cylinder charge. Thus, it is important to coordinate engine control inputs, such as fuel mass, injection timing, and intake and exhaust valve motion, to ensure robust auto-ignition combustion.
Timing of fuel injection is currently used to control ignition and combustion timing in diesel engines. Injection timing during the main compression stroke is the main alternative control option for controlling ignition timing in diesel engines. By retarding timing of injection, ignition of the cylinder charge is retarded. However, it is necessary to complete the injection of all fuel before ignition occurs.
It is well known that techniques for achieving low NOx emissions and particulates associated with diesel auto-ignition combustion are limited to moderate loads, because ignition of the cylinder charge is too rapid at high loads. It would be useful to have a method for retarding ignition timing that is not related to injection timing. It would be useful to expand the operating range of a compression-ignition engine in auto-ignition mode to improve emissions performance and fuel economy.
It is well known from studies of gasoline HCCI engines with negative valve overlap (NVO) that fuel injected during the negative valve overlap (recompression) advances combustion timing. The reason given for this is that the fuel injected during negative valve overlap is partially oxidized, or reformed, releasing some heat so that the temperature of the trapped residual is increased. The presence of higher trapped residuals results in a higher temperature at intake valve closing, leading to earlier ignition in a gasoline engine operating in HCCI mode.
At moderate and high loads in a premixed diesel engine, the ignition delay is decreased due to high in-cylinder temperature. As a result, combustion timing is over-advanced, resulting in excessive engine noise and possibly higher soot emissions. Premixed diesel combustion relies heavily on recirculated exhaust gas (EGR) to retard combustion. During transient operation of premixed diesel combustion, a delay in EGR reaching the cylinder can result in unstable ignition timing. At high load, engine noise becomes a problem because sufficient EGR cannot be inducted into the combustion chamber.
There is a need to control ignition timing independently of injection timing in a compression-ignition engine operating in an auto-ignition mode. The benefits of so operating include expanding the dynamic operating range of the engine in the auto-ignition mode, improving emissions performance and fuel economy, and, minimizing engine noise.